U.S. patent application number 15/461935 was filed with the patent office on 2017-09-28 for image forming apparatus.
The applicant listed for this patent is CANON KABUSHIKI KAISHA. Invention is credited to Masataka Mochizuki, Norihito Naito, Go Shindo.
Application Number | 20170277068 15/461935 |
Document ID | / |
Family ID | 58398074 |
Filed Date | 2017-09-28 |
United States Patent
Application |
20170277068 |
Kind Code |
A1 |
Naito; Norihito ; et
al. |
September 28, 2017 |
IMAGE FORMING APPARATUS
Abstract
When, in a state where a developer borne by a developer bearing
member is sandwiched by an opposing portion of an image bearing
member and the developer bearing member, C denotes capacitance
between the image bearing member and the developer bearing member,
.DELTA.V denotes a development contrast, Q/S denotes a charge
amount per unit area of the developer borne by the developer
bearing member, and .DELTA.v denotes a peripheral velocity ratio
which is a ratio of a peripheral velocity of the developer bearing
member to a peripheral velocity of the image bearing member, a
first peripheral velocity ratio is set so that
|Q/S.times..DELTA.v|.ltoreq.|C.times..DELTA.V| is satisfied, and a
second peripheral velocity ratio which is larger than the first
peripheral velocity ratio is set so that
|Q/S.times..DELTA.v|>|C.times..DELTA.V| is satisfied.
Inventors: |
Naito; Norihito;
(Numazu-shi, JP) ; Shindo; Go; (Mishima-shi,
JP) ; Mochizuki; Masataka; (Mishima-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CANON KABUSHIKI KAISHA |
Tokyo |
|
JP |
|
|
Family ID: |
58398074 |
Appl. No.: |
15/461935 |
Filed: |
March 17, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03G 15/065 20130101;
G03G 15/0815 20130101; G03G 15/0898 20130101; G03G 15/5037
20130101; G03G 15/5008 20130101; G03G 15/0266 20130101; G03G
15/0856 20130101 |
International
Class: |
G03G 15/06 20060101
G03G015/06; G03G 15/08 20060101 G03G015/08 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 22, 2016 |
JP |
2016-057626 |
Claims
1. An image forming apparatus, comprising: an image bearing member;
a developer bearing member configured to perform a development
operation in which an electrostatic image formed on the image
bearing member is developed with a developer; driving means
configured to rotationally drive the image bearing member and the
developer bearing member respectively so that the peripheral
velocities of each is variable individually; latent image forming
means configured to form an electrostatic image on the image
bearing member by forming a light-part potential and a dark-part
potential on the image bearing member; and applying means
configured to apply a developing bias to the developer bearing
member, wherein when a peripheral velocity ratio is defined as a
ratio of the peripheral velocity of the developer bearing member to
the peripheral velocity of the image bearing member, the driving
means is configured to be capable of driving the image bearing
member and the developer bearing member at a first peripheral
velocity ratio and a second peripheral velocity ratio which is
larger than the first peripheral velocity ratio, and when C denotes
capacitance between the image bearing member and the developer
bearing member in a state that the developer is sandwiched between
the image bearing member and the developer bearing member while the
developer is supplied to the image bearing member from the
developer bearing member, .DELTA.V denotes a development contrast
which is a potential difference between the light-part potential
and the developing bias, Q/S denotes a charge amount per unit area
of the developer borne by the developer bearing member, and
.DELTA.v denotes the peripheral velocity ratio, the first
peripheral velocity ratio is set so that a relationship expressed
by |Q/S.times..DELTA.v|.ltoreq.|C.times..DELTA.V| is satisfied, and
the second peripheral velocity ratio is set so that a relationship
expressed by |Q/S.times..DELTA.v|>|C.times..DELTA.V| is
satisfied.
2. The image forming apparatus according to claim 1, wherein the
first peripheral velocity ratio and the second peripheral velocity
ratio are set so that an amount of the developer remaining on the
developer bearing member after the development operation in a case
where the development operation is performed at the second
peripheral velocity ratio is larger than that in a case where the
development operation is performed at the first peripheral velocity
ratio.
3. The image forming apparatus according to claim 1, further
comprising detecting means configured to detect temperature and
humidity, wherein when a temperature detected by the detecting
means is equal to or lower than a prescribed temperature and a
humidity detected by the detecting means is equal to or lower than
a prescribed humidity, the latent image forming means changes the
light-part potential or the applying means changes a magnitude of
the developing bias to be applied so that a second development
contrast which is smaller than a first development contrast is
formed.
4. The image forming apparatus according to claim 3, wherein when a
temperature detected by the detecting means is equal to or higher
than a prescribed temperature and a humidity detected by the
detecting means is equal to or higher than a prescribed humidity,
the latent image forming means changes the light-part potential so
that a third development contrast which is larger than the first
development contrast is formed.
5. The image forming apparatus according to claim 1, wherein the
latent image forming means includes: charging means configured to
charge the image bearing member to form the dark-part potential on
the image bearing member, and exposing means configured to expose
the charged image bearing member to form the light-part potential
on the image bearing member, wherein the light-part potential is
changed by changing an amount of light for exposure by the exposing
means.
6. The image forming apparatus according to claim 1, wherein the
driving means is configured to rotate the image bearing member and
the developer bearing member so that the image bearing member and
the developer bearing member move in a same direction at an
opposing portion where the image bearing member and the developer
bearing member oppose each other.
7. The image forming apparatus according to claim 1, wherein the
driving means is configured to: set the peripheral velocity of the
image bearing member in the case of the first peripheral velocity
ratio same as that in the case of the second peripheral velocity
ratio; and set the peripheral velocity of the developer bearing
member higher in the case of the second peripheral velocity ratio
than that in the case of the first peripheral velocity ratio.
8. The image forming apparatus according to claim 1, wherein the
driving means is configured to: set the peripheral velocity of the
developer bearing member in the case of the first peripheral
velocity ratio same as that in the case of the second peripheral
velocity ratio; and set the peripheral velocity of the image
bearing member lower in the case of the second peripheral velocity
ratio than that in the case of the first peripheral velocity
ratio.
9. The image forming apparatus according to claim 1, wherein the
driving means is configured to make the second peripheral velocity
ratio larger than the first peripheral velocity ratio by: setting
the peripheral velocity of the developer bearing member lower in
the case of the second peripheral velocity ratio than that in the
case of the first peripheral velocity ratio; setting the peripheral
velocity of the image bearing member lower in the case of the
second peripheral velocity ratio than that in the case of the first
peripheral velocity ratio; wherein an amount of reduction of the
peripheral velocity of the developer bearing member is different
from an amount of reduction of the peripheral velocity of the image
bearing member.
10. The image forming apparatus according to claim 1, wherein the
first peripheral velocity ratio and the second peripheral velocity
ratio are set so that an image can be formed at the second
peripheral velocity ratio on a recording material longer than the
one on which an image is formed at the first peripheral velocity
ratio.
11. The image forming apparatus according to claim 1, wherein the
first peripheral velocity ratio and the second peripheral velocity
ratio are set so that a laid-on level of the developer per unit
area of an image formed on a recording material in a case of the
second peripheral velocity ratio is higher than that in a case of
the first peripheral velocity ratio.
12. The image forming apparatus according to claim 1, further
comprising: a supplying member configured to supply a developer to
the developer bearing member; a developing chamber in which the
supplying member is arranged; a housing chamber configured to
communicate with the developing chamber and house the developer;
and a conveying member arranged in the housing chamber and
configured to convey the developer toward the developing chamber,
wherein a communication port through which the developing chamber
and the housing chamber communicate is positioned above the
conveying member in the housing chamber.
13. An image forming apparatus, comprising: an image bearing
member; a developer bearing member configured to perform a
development operation in which an electrostatic image formed on the
image bearing member is developed with a developer; and driving
means configured to rotationally drive the image bearing member and
the developer bearing member respectively so that the peripheral
velocities of each is variable individually, wherein when a
peripheral velocity ratio is defined as a ratio of the peripheral
velocity of the developer bearing member to the peripheral velocity
of the image bearing member, the driving means is configured to be
capable of driving the image bearing member and the developer
bearing member at a first peripheral velocity ratio and a second
peripheral velocity ratio which is larger than the first peripheral
velocity ratio, and wherein the first peripheral velocity ratio and
the second peripheral velocity ratio are set so that an amount of
the developer remaining on the developer bearing member after the
development operation in a case where the development operation is
performed at the second peripheral velocity ratio is larger than
that in a case where the development operation is performed at the
first peripheral velocity ratio.
Description
BACKGROUND OF THE INVENTION
[0001] Field of the Invention
[0002] The present invention relates to an image forming apparatus
using an electrophotographic system.
[0003] Description of the Related Art
[0004] Conventionally, as an image forming apparatus such as a
laser beam printer, an image forming apparatus is known which
adopts an in-line color system and which is constituted by a
plurality of image forming stations in which a photosensitive drum
as an image bearing member is arranged in plurality in a rotation
direction of an intermediate transfer member. In the image forming
apparatus, an electrostatic latent image created on the
photosensitive drum in each of the plurality of image forming
stations is developed into a toner image by developing means, and
the resultant is primarily transferred to the intermediate transfer
member. By similarly repeating primary transfers in the plurality
of image forming stations in this step, a full-color toner image is
formed on the intermediate transfer member. Subsequently, the
full-color toner image is secondarily transferred to recording
material and, furthermore, the full-color toner image is fixed to
the recording material by fixing means. An image formed by the
series of image forming operations must represent output of an
image and density intended by a user. In addition, a full-color
image created by the plurality of image forming stations requires
tinge reproducibility and stability.
[0005] In consideration thereof, Japanese Patent Application
Laid-open No. H8-227222 proposes a method of increasing a tinge
selection range by changing a developing bias or a rotational speed
of a developing roller as a developer bearing member or the like
according to purpose. In addition, Japanese Patent Application
Laid-open No. 2013-210489 proposes a method for overcoming problems
such as toner scattering and image thinning which accompany an
increase in a tinge selection range or an improvement in density.
This method enables an image with an increased tinge selection
range or a high-density image to be output without causing
image-related problems by reducing a peripheral velocity of a
photosensitive drum to increase a peripheral velocity ratio between
the photosensitive drum and a developing roller. Furthermore, in a
case of high-density printing such as solid black, by forming a
development contrast such that all of toner on a developing roller
is developed onto a photosensitive drum, a tinge selection range is
increased, high density is realized, and stabilization is provided
while minimizing an effect of a potential fluctuation of the
photosensitive drum and the like.
SUMMARY OF THE INVENTION
[0006] As described above, Japanese Patent Application Laid-open
No. H8-227222 and Japanese Patent Application Laid-open No.
2013-210489 enable a tinge selection range to be increased and
high-density printing to be performed by increasing a toner supply
amount from a developing roller to a photosensitive drum. However,
when an increase in a tinge selection range and high-density output
as described in Japanese Patent Application Laid-open No. H8-227222
and Japanese Patent Application Laid-open No. 2013-210489 are
consecutively performed in addition to normal printing operations,
consumption of toner of the developing roller is accelerated as the
toner is developed onto the photosensitive drum and a toner supply
amount to the developing roller itself may run short. It is found
that a shortage of the toner supply amount to the developing roller
may result in an image containing image density non-uniformity and
tinge variations and may prevent an intended image from being
obtained.
[0007] An object of the present invention is to provide a technique
for reducing effects of image density non-uniformity, color
non-uniformity, and the like on an output image.
[0008] In order to achieve the object described above, an image
forming apparatus according to an embodiment of the present
invention is an image forming apparatus, comprising:
[0009] an image bearing member;
[0010] a developer bearing member configured to perform a
development operation in which an electrostatic image formed on the
image bearing member is developed with a developer;
[0011] driving means configured to rotationally drive the image
bearing member and the developer bearing member, respectively so
that the peripheral velocities of each is variable
individually;
[0012] latent image forming means configured to form an
electrostatic image on the image bearing member by forming a
light-part potential and a dark-part potential on the image bearing
member; and
[0013] applying means configured to apply a developing bias to the
developer bearing member, wherein
[0014] when a peripheral velocity ratio is defined as a ratio of
the peripheral velocity of the developer bearing member to the
peripheral velocity of the image bearing member, the driving means
is configured to be capable of driving the image bearing member and
the developer bearing member at a first peripheral velocity ratio
and a second peripheral velocity ratio which is larger than the
first peripheral velocity ratio, and when
[0015] C denotes capacitance between the image bearing member and
the developer bearing member in a state that the developer is
sandwiched between the image bearing member and the developer
bearing member while the developer is supplied to the image bearing
member from the developer bearing member,
[0016] .DELTA.V denotes a development contrast which is a potential
difference between the light-part potential and the developing
bias,
[0017] Q/S denotes a charge amount per unit area of the developer
borne by the developer bearing member, and
[0018] .DELTA.v denotes the peripheral velocity ratio,
[0019] the first peripheral velocity ratio is set so that a
relationship expressed by
|Q/S.times..DELTA.v|.ltoreq.|C.times..DELTA.V| is satisfied,
and
[0020] the second peripheral velocity ratio is set so that a
relationship expressed by
|Q/S.times..DELTA.v|>|C.times..DELTA.V| is satisfied.
[0021] In order to achieve the object described above, an image
forming apparatus according to an embodiment of the present
invention is an image forming apparatus, comprising:
[0022] an image bearing member;
[0023] a developer bearing member configured to perform a
development operation in which an electrostatic image formed on the
image bearing member is developed with a developer; and
[0024] driving means configured to rotationally drive the image
bearing member and the developer bearing member respectively so
that the peripheral velocities of each is variable individually,
wherein
[0025] when a peripheral velocity ratio is defined as a ratio of
the peripheral velocity of the developer bearing member to the
peripheral velocity of the image bearing member, the driving means
is configured to be capable of driving the image bearing member and
the developer bearing member at a first peripheral velocity ratio
and a second peripheral velocity ratio which is larger than the
first peripheral velocity ratio, and wherein
[0026] the first peripheral velocity ratio and the second
peripheral velocity ratio are set so that an amount of the
developer remaining on the developer bearing member after the
development operation in a case where the development operation is
performed at the second peripheral velocity ratio is larger than
that in a case where the development operation is performed at the
first peripheral velocity ratio.
[0027] According to the present invention, effects of image density
non-uniformity, color non-uniformity, and the like on an output
image can be reduced.
[0028] Further features of the present invention will become
apparent from the following description of exemplary embodiments
with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIGS. 1A and 1B are explanatory diagrams of a peripheral
velocity ratio and a state of toner on a photosensitive drum
according to an embodiment of the present invention;
[0030] FIG. 2 is a schematic view of an image forming apparatus
according to first and second embodiments of the present
invention;
[0031] FIG. 3 is a schematic view of a process cartridge according
to an embodiment of the present invention;
[0032] FIG. 4 is an explanatory diagram of an amount of toner on
paper and density according to an embodiment of the present
invention;
[0033] FIG. 5 is a chromaticity diagram according to an embodiment
of the present invention;
[0034] FIG. 6 is a flow chart according to a third embodiment of
the present invention;
[0035] FIG. 7 is a schematic view of an image forming apparatus
according to third and fourth embodiments of the present
invention;
[0036] FIG. 8 is a block diagram according to the third and fourth
embodiments of the present invention;
[0037] FIG. 9 is a characteristic diagram of a peripheral velocity
of a developing roller with respect to a photosensitive drum and a
toner coating amount of the developing roller;
[0038] FIG. 10 is a characteristic diagram of a toner coating
amount [kg/m.sup.2] on a developing roller and image formation
density;
[0039] FIG. 11 is a characteristic diagram of a toner coating
amount and a toner charge amount of a developing roller; and
[0040] FIG. 12 is a flow chart according to the fourth embodiment
of the present invention.
DESCRIPTION OF THE EMBODIMENTS
[0041] Modes for carrying out the present invention are
illustratively explained in detail below on the basis of embodiment
with reference to the drawings. However, dimensions, materials, and
shapes of components described in the embodiments, relative
arrangement of the components, and the like should be changed as
appropriate according to the configuration of an apparatus to which
the invention is applied and various conditions. That is, the
dimensions, the materials, the shapes, and the relative arrangement
are not intended to limit the scope of the present invention to the
embodiments.
First Embodiment
[0042] An image forming apparatus according to the present
embodiment has two image formation modes: an image formation mode A
for obtaining normal image density; and an image formation mode B
for obtaining high density or increasing a tinge selection range by
changing a peripheral velocity ratio between a photosensitive drum
as an image bearing member and a developing roller as a developer
bearing member. Each image formation mode has a different ratio of
rotational speed (a peripheral velocity ratio) between a
photosensitive drum and a developing roller particularly under a
condition of forming a solid black image. In the image formation
mode A, with respect to a development contrast formed by an
electrostatic latent image formed on the photosensitive drum and a
developing bias applied to the developing roller, all of the toner
on the developing roller is developed onto the photosensitive drum.
In the image formation mode B, the peripheral velocity ratio
between the photosensitive drum and the developing roller is
increased to increase a toner supply amount from the developing
roller to the photosensitive drum. In addition, by reducing or
canceling an electrical gradient created by the development
contrast by a charge of the toner having been imparted a charge on
the developing roller, a part of the toner on the developing roller
is retained on the developing roller instead of being transferred
to the photosensitive drum.
[0043] [Image Forming Apparatus]
[0044] An image forming apparatus according to an embodiment of the
present invention will now be described with reference to FIG. 2
using an electrophotographic system as an example. FIG. 2 is a
schematic sectional view of an image forming apparatus 200
according to the present embodiment. The image forming apparatus
200 according to the present embodiment is a full-color laser
printer adopting an in-line system and an intermediate transfer
system. The image forming apparatus 200 is configured to be capable
of forming a full-color image on recording material (for example,
recording paper, a plastic sheet, and cloth) in accordance with
image information. The image information is input to a CPU 215
provided in an engine controller 214 from an image reading
apparatus connected to the image forming apparatus 200 or from a
host device (not shown) such as a personal computer which is
connected to the image forming apparatus 200 so as to be capable of
communication.
[0045] As a plurality of image forming portions, the image forming
apparatus 200 includes first, second, third, and fourth image
forming stations SY, SM, SC, and SK for respectively forming images
of the colors yellow (Y), magenta (M), cyan (C), and black (K). In
this case, an image forming station is constituted by a process
cartridge 208 and a primary transfer roller 212 arranged on an
opposite side via an intermediate transfer belt 205. In the present
embodiment, the first to fourth image forming portions SY, SM, SC,
and SK are arranged in a single row in a direction intersecting a
vertical direction. Moreover, in the present embodiment,
configurations and operations of the first to fourth image forming
portions are substantially the same with the exception of
differences in colors of formed images. Therefore, unless the image
forming portions must be distinguished from one another, the
suffixes Y, M, C, and K added to the reference characters to
represent which color is to be produced by which element will be
omitted and the image forming portions will be collectively
described. Alternatively, a configuration may be adopted in which a
process cartridge for black which is used more frequently has a
larger size than other process cartridges.
[0046] The process cartridge 208 is configured to be attachable and
detachable to and from an image forming apparatus main body
(hereinafter, an apparatus main body) via mounting means such as a
mounting guide or a positioning member provided on the apparatus
main body. In this case, the apparatus main body refers to an
apparatus constituent portion of the configuration of the image
forming apparatus 200 from at least the process cartridge 208 is
excluded. Alternatively, a configuration in which a developing unit
204 (to be described later) is independently attachable and
detachable to and from an apparatus main body may be adopted, in
which case an apparatus constituent portion of the configuration of
the image forming apparatus 200 excluding the developing unit 204
may be considered the apparatus main body.
[0047] As a plurality of image bearing members, the image forming
apparatus 200 includes four drum-shaped electrophotographic
photoreceptors or, in other words, four photosensitive drums 201
arranged parallel to each other in a direction intersecting the
vertical direction. The photosensitive drum 201 is rotationally
driven in a direction of an illustrated arrow A (clockwise) by a
motor drive portion 404 shown in FIG. 3 as driving means (a drive
source). A charging roller 202 is charging means configured to
uniformly charge a surface of the photosensitive drum 201. A
scanner unit (an exposing apparatus) 203 is exposing means
configured to irradiate a laser based on image information to form
an electrostatic image (an electrostatic latent image) on the
photosensitive drum 201, and includes lasers 217 in a number
corresponding to the number of the photosensitive drums 201. The
developing unit (a developing apparatus) 204 is developing means
configured to develop an electrostatic image as a toner image. A
cleaning blade 206 is cleaning means configured to remove toner
(untransferred toner) remaining on a surface of the photosensitive
drum 201 after transfer, and a preliminary exposure LED 216
eliminates a potential on the photosensitive drum 201. The
intermediate transfer belt 205 is arranged so as to oppose the four
photosensitive drums 201 and functions as an intermediate transfer
member for transferring a toner image on the photosensitive drum
201 to a recording material 207. The process cartridge 208 is
integrally constituted by the photosensitive drum 201, the charging
roller 202 as charging process means of the photosensitive drum
201, the developing unit 204, and the cleaning blade 206 and is
configured so as to be attachable and detachable to and from the
image forming apparatus 200. In the present embodiment, all of the
process cartridges 208 for the respective colors have a same shape,
and toners of the respective colors of yellow (Y), magenta (M),
cyan (C), and black (K) are housed in the respective process
cartridges 208. In addition, the toners used in the present
embodiment are toners having negative-charging characteristics (of
which a normal charging polarity is negative).
[0048] The intermediate transfer belt 205 as an intermediate
transfer member formed by an endless belt is in contact with all
photosensitive drums 201 as image bearing members and rotates in a
direction of an illustrated arrow B (counterclockwise). The
intermediate transfer belt 205 is stretched over a plurality of
supporting members including a driver roller 209, a secondary
transfer opposing roller 210, and a driven roller 211. Four primary
transfer rollers 212 as primary transfer means are arranged
parallel to each other on a side of an inner peripheral surface of
the intermediate transfer belt 205 so as to oppose each
photosensitive drum 201. In addition, a bias having an opposite
polarity to the normal charging polarity (in the present
embodiment, negative polarity as described earlier) of the toners
is applied to the primary transfer rollers 212 from a primary
transfer bias power supply (not shown). Accordingly, a toner image
on the photosensitive drum 201 is transferred onto the intermediate
transfer belt 205. In addition, a secondary transfer roller 213 as
secondary transfer means is arranged at a position opposing the
secondary transfer opposing roller 210 on a side of an outer
peripheral surface of the intermediate transfer belt 205.
Furthermore, a bias having an opposite polarity to the normal
charging polarity of the toners is applied to the secondary
transfer roller 213 from a secondary transfer bias power supply
(not shown). Accordingly, a toner image on the intermediate
transfer belt 205 is transferred onto the recording material
207.
[0049] The recording material 207 onto which the toner image has
been transferred is conveyed to a fixing apparatus 218 as fixing
means. Heat and pressure are applied to the recording material 207
by the fixing apparatus 218 to fix the toner image onto the
recording material 207. Subsequently, the recording material 207
onto which the toner image has been fixed is discharged to a paper
discharge tray provided on an upper surface of the apparatus main
body.
[0050] [Process Cartridge]
[0051] The process cartridge 208 to be mounted to the image forming
apparatus 200 according to the present embodiment will now be
described with reference to FIG. 3. FIG. 3 is a sectional (main
sectional) view schematically showing a cross section perpendicular
to a longitudinal direction (a rotational axis direction) of the
photosensitive drum 201 as an image bearing member. In the present
embodiment, configurations and operations of the process cartridges
208 of the respective colors are substantially the same with the
exception of types (colors) of developers housed therein.
[0052] The process cartridge 208 includes a photoreceptor unit 301
including the photosensitive drum 201 as an image bearing member
and the like and the developing unit 204 including a developing
roller 302 and the like. The photoreceptor unit 301 includes a
cleaning frame body 303 as a frame body that supports various
elements in the photoreceptor unit 301. The photosensitive drum 201
is rotatably attached to the cleaning frame body 303 via a bearing
member (not shown). The photosensitive drum 201 is rotationally
driven in the direction of the illustrated arrow A (clockwise) in
accordance with an image forming operation as a driving force of
the motor drive portion 404 as driving means (a drive source) is
transferred to the photoreceptor unit 301. As the photosensitive
drum 201 which is to perform a central role of an image forming
process, an organic photoreceptor is used in which an outer
circumferential surface of an aluminum cylinder is sequentially
coated with an undercoat layer, a carrier generation layer, and a
carrier transfer layer which are functional membranes. In addition,
the cleaning blade 206 and the charging roller 202 are arranged in
the photoreceptor unit 301 so as to come into contact with a
circumferential surface of the photosensitive drum 201.
Untransferred toner removed from the surface of the photosensitive
drum 201 by the cleaning blade 206 is dropped into and housed in
the cleaning frame body 303.
[0053] The charging roller 202 which is charging means is driven to
rotate when a roller portion made of conductive rubber is brought
into pressure contact with the photosensitive drum 201 as an image
bearing member. In a core of the charging roller 202, as a charging
step, prescribed DC voltage as a charging bias is applied to the
photosensitive drum 201 from a charging voltage applying portion
(high-voltage power supply) 401 as charging roller bias applying
means. Accordingly, a uniform dark-part potential (Vd) is formed on
the surface of the photosensitive drum 201. The scanner unit 203
described earlier emits laser light L corresponding to image data
to expose the photosensitive drum 201. In the exposed
photosensitive drum 201, charges on the surface are eliminated by a
carrier from the carrier generation layer and the potential drops.
As a result, an electrostatic latent image in which an exposed
portion has a prescribed light-part potential (Vl) and an unexposed
portion has a prescribed dark-part potential (Vd) is formed on the
photosensitive drum 201. In the electrostatic latent image, a zone
in which the light-part potential is formed is a zone to which
toner is to be adhered and a zone in which the dark-part potential
is formed is a zone to which toner is not to be adhered.
[0054] The developing unit 204 includes a container frame body 306
having a developing chamber 18a and a developer housing chamber
18b. The developer housing chamber 18b is arranged below the
developing chamber 18a and communicates with the developing chamber
18a via a communication port provided in an upper part of the
developer housing chamber 18b. Toner 305 as a developer is housed
inside the developer housing chamber 18b. In addition, the
developer housing chamber 18b is provided with a stirring member
(developer conveying member) 307 for conveying the toner 305 to the
developing chamber 18a. The stirring member 307 conveys the toner
305 to the developing chamber 18a by rotating in a direction of an
illustrated arrow G. The stirring member 307 rotates by obtaining a
rotational driving force from a motor drive portion 406 as driving
means. Moreover, in the present embodiment, toner of which a normal
charging polarity is negative is used as the toner 305 as described
above. Accordingly, the following description assumes the use of a
negative-charging toner. However, toners usable in the present
invention are not limited to a negative-charging toner and,
depending on apparatus configuration, toner of which a normal
charging polarity is positive may be used.
[0055] The developing chamber 18a is provided with the developing
roller 302 as a developer bearing member which comes into contact
with the photosensitive drum 201 as an image bearing member and
which rotates in a direction of an illustrated arrow D by receiving
a driving force from a motor drive portion 403 as driving means. In
the present embodiment, the developing roller 302 and the
photosensitive drum 201 respectively rotate so that respective
surfaces thereof move in a same direction in an opposing portion (a
contact portion C1) which is a portion where the toner 305 borne by
the developing roller 302 is supplied to the photosensitive drum
201. In addition, a prescribed DC bias (developing bias) sufficient
to develop and visualize an electrostatic latent image on the
photosensitive drum 201 as a toner image (developer image) is
applied to the developing roller 302 from a developing voltage
applying portion (high-voltage power supply) 402 as developing bias
applying means. The electrostatic latent image is visualized in the
contact portion C1 where the developing roller 302 and the
photosensitive drum 201 are in contact with each other by
transferring toner only to portions with the light-part potential
using a potential difference between the developing roller 302 and
the photosensitive drum 201.
[0056] A toner supplying roller (hereinafter, a supplying roller)
304 and a developing blade (hereinafter, a restricting member) 303
which is a toner amount restricting member are further arranged in
the developing chamber 18a. The supplying roller 304 as a developer
supplying member is a roller for supplying the toner 305 conveyed
from the developer housing chamber 18b to the developing roller
302. The restricting member 303 restricts a coating amount of, and
imparts charges to, the toner on the developing roller 302 supplied
by the supplying roller 304. A bias (supplying bias) is applied to
the supplying roller 304 from a high-voltage power supply (not
shown) as supplying bias applying means.
[0057] In this case, the biases applied by the developing voltage
applying portion (high-voltage power supply) 402, the charging
voltage applying portion (high-voltage power supply) 401, and the
supplying roller bias power supply are controlled by the CPU 215
which is a controller based on information obtained by a printing
mode information acquiring portion 70. The printing mode
information acquiring portion 70 acquires information input using
an operating panel (not shown) of the image forming apparatus 200,
information input from a printer driver, and the like.
[0058] The supplying roller 304 is an elastic sponge roller in
which a foam layer is formed on an outer periphery of a conductive
core and is arranged so as to form a prescribed contact portion C2
on a circumferential surface of the developing roller 302 in an
opposing portion with the developing roller 302. In addition, by
receiving a driving force from a motor drive portion 405 as driving
means, the supplying roller 304 rotates in a direction of an
illustrated arrow E. Moreover, in the present embodiment, the motor
drive portions 404, 403, 405, and 406 which drive the
photosensitive drum 201, the developing roller 302, the supplying
roller 304, and the stirring member 307 are respectively
constituted by a motor, a gear train which transmits a rotational
driving force of the motor, and the like. The motor drive portions
404, 403, 405, and 406 correspond to driving means capable of
rotationally driving, in individual and variable fashion, an image
bearing member, a developer bearing member, a supplying member, and
a conveying member according to the present embodiment and are
controlled by the CPU 215. Furthermore, the drive configuration
shown in FIG. 3 applies to process cartridges of yellow (Y),
magenta (M), and cyan (C). In other words, a configuration is
adopted where driving means configured to rotationally drive a
photosensitive drum, driving means configured to rotationally drive
a developing roller, driving means configured to rotationally drive
a supplying roller, and driving means configured to rotationally
drive a stirring member respectively have different driving sources
(driving motors). In a process cartridge 208K of black (K), driving
means configured to rotationally drive a photosensitive drum,
driving means configured to rotationally drive a developing roller,
driving means configured to rotationally drive a supplying roller,
and driving means configured to rotationally drive a stirring
member are constituted by a single shared driving motor.
[0059] FIG. 4 is a characteristic diagram showing a relationship
between a supply amount [kg/m.sup.2] of the toner 305 from the
developing roller 302 as a developer bearing member to the
photosensitive drum 201 as an image bearing member and image
formation density, in which a horizontal axis represents a toner
amount on paper (on recording material) and a vertical axis
represents the density after fixing. In the configuration described
above, performing image formation may cause a developed toner
amount to fluctuate due to variations in potential among various
biases and the like. Image defects such as image density
non-uniformity and tinge non-uniformity may occur in an image
formed when the toner amount fluctuates. FIG. 4 represents an
example thereof. Moreover, the characteristic diagram shown in FIG.
4 is obtained using a reflection densitometer (Macbeth RD-918)
manufactured by X-Rite GmbH (previously GretagMacbeth GmbH) as a
reflection densitometer. As determination criteria of the image
density, for example, an average density of a solid image of 1.3 or
higher may be required in an output image of a high-image quality
image forming apparatus.
[0060] FIG. 4 shows that, when the toner amount ranges from 0 to
around 1.2, a change in density after fixing becomes steep and
image density non-uniformity may occur due to a variation in the
toner amount. An effective method for avoiding image density
non-uniformity is to develop the electrostatic image on the
photosensitive drum using all of a coat of toner on the developing
roller which is formed in a relatively stable manner. To this end,
as a development setting when developing a high printing image
pattern such as a solid black image, a setting which attains a
large absolute value of a potential difference between a light-part
potential and a developing bias applied to the developing roller (a
large development contrast) is adopted. By forming a latent image
having such a sufficient development contrast, even when developing
performance varies due to potential fluctuation or the like, a
stable toner image-developed image can be obtained. Moreover, a
configuration involved with the formation of a development contrast
in the present embodiment or, in other words, the charging roller
202, the charging voltage applying portion 401, the scanner unit
203, the developing roller 302, the developing voltage applying
portion 402, and the like correspond to latent image forming means
according to the present invention.
[0061] One of the wide varieties of market needs is to realize
higher image density and increased tinge to enable images with
enhanced colorfulness to be obtained. To this end, in addition to a
mode for obtaining general image density, an operating mode for
increasing a toner supply amount to a photosensitive drum by
changing a peripheral velocity ratio between the photosensitive
drum and a developing roller is proposed as a mode for realizing
high density and increased tinge. Examples of a method of
increasing the peripheral velocity ratio include increasing a
rotational speed of the developing roller and reducing a rotational
speed of the photosensitive drum. For example, the peripheral
velocity ratio may be changed by reducing both the rotational speed
of the developing roller and the rotational speed of the
photosensitive drum and, at the same time, differentiating amounts
of reduction thereof. In addition, a print image realizing higher
density and increased tinge is known to consume a relatively large
amount of toner. In many cases, printing conditions for realizing
high density and a wide color gamut are set as one of a plurality
of selectable modes together with printing conditions for realizing
general image quality used in offices and the like, in which case a
user can optionally select one of the modes.
[0062] It was found that, when a photosensitive drum and a
developing roller are operated at a large peripheral velocity ratio
in order to obtain a high-density image and high-density images are
consecutively output, the supply of toner is sometimes unable to
keep up with the consecutive output and image density
non-uniformity or tinge non-uniformity may occur. A conceivable
cause is as follows. Specifically, when images are consecutively
output in a high-density mode which uses toner in large amounts,
although the toner itself is supplied to a vicinity of the
developing roller, since the supplied toner has fewer opportunities
to be imparted with charges, an electrified charge of the toner is
weak. Therefore, the supply of toner including toner adhesion to
the developing roller is easily destabilized. When supply becomes
unstable, a toner coating amount on the developing roller or an
electrified charge amount of the toner also becomes unstable. As a
result, image density non-uniformity and tinge non-uniformity occur
in an image.
[0063] In consideration thereof, in the present embodiment, the
following operations are performed in an apparatus configuration
including, as operating modes of image formation, a normal mode (an
image formation mode A) for office applications and the like and a
high-density mode (an image formation mode B) for realizing high
density and increased tinge. In the normal mode as a first
operating mode, a setting condition is adopted so as to have a
development contrast resulting in a residual toner amount
immediately after a development operation of almost zero with
respect to an electrified charge amount per unit area of the toner
supplied from the developing roller under a printing condition with
solid black density. In the high-density mode as a second operating
mode, while increasing a toner supply amount by increasing the
peripheral velocity ratio between the photosensitive drum and the
developing roller under the same condition is similar to
conventional methods, the high-density mode differs from
conventional methods in the way the peripheral velocity ratio is
increased. In the present embodiment, the peripheral velocity ratio
is increased so that the electrified charge amount of the toner
supplied from the developing roller further increases with respect
to the development contrast and the toner remains on the developing
roller or a residual toner amount on the developing roller further
increases immediately after a development operation or, in other
words, after the developing roller passes the opposing portion with
the photosensitive drum. Accordingly, density and tinge are
increased and, at the same time, image density non-uniformity and
tinge non-uniformity are suppressed.
[0064] First, an electrostatic latent image formed on a
photosensitive drum and an electrified charge amount of toner will
be confirmed. In the present embodiment, a dark-part potential
after charging of -500 V and a light-part potential after laser
exposure of -100 V are assumed. In the present embodiment, a value
of the light-part potential refers to a value of a measurement
performed by a surface potentiometer on a photosensitive drum when
forming an image pattern involving developing an entire sheet of
paper with toner as in the case of a solid black image. A
developing potential (a developing bias) applied to the developing
roller is assumed to be -300 V and a development contrast .DELTA.V
upon the application of the developing potential is assumed to be
200 V. With respect to the toner formed on the developing roller,
in the present embodiment, a toner laid-on level per unit area
(hereinafter, referred to as M/S) is set to 3.0.times.10.sup.-3
kg/m.sup.2 and an electrified charge amount of the toner per unit
area (hereinafter, referred to as Q/S) is set to
-0.15.times.10.sup.-3 C/m.sup.2.
[0065] A toner supply amount with respect to a development contrast
will be confirmed. The confirmation was performed by setting a
peripheral velocity of the photosensitive drum to 0.2 m/s and
varying a peripheral velocity of the developing roller to change a
peripheral velocity ratio which is a ratio of the peripheral
velocity of the developing roller to the peripheral velocity of the
photosensitive drum. In this case, with 100% representing uniform
velocity, the peripheral velocity ratio signifies that, for
example, the developing roller rotates faster than the
photosensitive drum at 140%. Alternatively, a configuration may be
adopted in which the peripheral velocity ratio is increased by
fixing the peripheral velocity of the developing roller to a
constant velocity of 0.2 m/s and reducing the peripheral velocity
of the photosensitive drum. In addition, since tinge and density
are strongly related to each other, the present embodiment will be
described using density. Furthermore, black toner was used as the
toner in the present evaluation. Results thereof are shown in FIGS.
1A and 1B.
[0066] FIG. 1A shows the peripheral velocity ratio on a vertical
axis and M/S developed on the photosensitive drum as an image
bearing member on a horizontal axis. FIG. 1B similarly shows the
peripheral velocity ratio on a vertical axis and Q/S of toner
developed on the photosensitive drum on a horizontal axis. FIGS. 1A
and 1B show that increases in M/S and Q/S relative to the
peripheral velocity ratio slow down around a peripheral velocity
ratio of 210%. In addition, a relationship between the peripheral
velocity ratio when the development contrast .DELTA.V is set to 150
V and M/S or Q/S is depicted by a dashed line. The slowdown
indicates that, when charged toner is supplied to the
photosensitive drum, an electrical gradient formed by the
development contrast is moderated or eliminated by the charge of
the toner and the supply of toner to portions with a light-part
potential of the photosensitive drum enters a saturated state.
[0067] A development contrast in a developing nip portion is formed
by a dark-part potential and a light-part potential constituting an
electrostatic latent image formed on the photosensitive drum and by
a developing bias applied to the developing roller. Due to the
development contrast, the toner on the developing roller is
transferred to the photosensitive drum and develops the
electrostatic image. An amount of toner supplied for development (a
developable amount) due to the development contrast is determined
by a product of capacitance (C) between the photosensitive drum and
the developing roller at the developing nip portion which
sandwiches the toner and the development contrast (.DELTA.V) with
respect to a total electrified charge amount of the supplied toner.
In other words, C.times..DELTA.V represents a total electrified
charge amount of the toner per unit area which can be transferred
from the developing roller to the photosensitive drum (which can be
supplied for development) in the developing nip portion which is an
opposing portion where the developing roller and the photosensitive
drum oppose each other. In addition, the total electrified charge
amount of the toner supplied to the photosensitive drum is
determined in accordance with the electrified charge amount (Q/S)
per unit area on the developing roller and a peripheral velocity
ratio (.DELTA.v) with respect to the photosensitive drum and is
expressed as a product Q/S.times..DELTA.v.
[0068] From the above, an amount of toner which can be supplied for
development with respect to the development contrast can be
expressed by a relational expression of
|Q/S.times..DELTA.v|=|C.times..DELTA.V|. In other words, when the
peripheral velocity ratio .DELTA.v is varied and
|Q/S.times..DELTA.v|.ltoreq.|C.times..DELTA.V| is satisfied, a
total charge amount of the toner supplied from the developing
roller is smaller than a charge amount which can be accepted by the
photosensitive drum. This case constitutes a condition under which
all of the toner on the developing roller is transferred to the
photosensitive drum (supplied for development). Conversely, when
|Q/S.times..DELTA.v|>C.times..DELTA.V| is satisfied, the total
charge amount of the toner supplied from the developing roller is
larger than the charge amount which can be accepted by the
photosensitive drum. This case constitutes a condition under which
while a part of the toner corresponding to the charge amount which
satisfies |Q/S.times..DELTA.v|=|C.times..DELTA.V| is used for
development, the rest of the toner remains on the developing roller
after (immediately after) the development operation instead of
being used for development.
[0069] As shown in FIGS. 1A and 1B, when .DELTA.V=200 V, M/S on the
photosensitive drum slows down under a condition of .DELTA.v=210
[%] and Q/S.times..DELTA.v takes a value around
-0.32.times.10.sup.-3. Therefore, from the relationship expressed
as |Q/S.times..DELTA.v|=|C.times..DELTA.V|, the capacitance C which
is a product of a capacity between the photosensitive drum and the
developing roller takes a value of 1.6.times.10.sup.-6.
[0070] The toner image developed on the photosensitive drum is
eventually transferred to and fixed on recording material. FIG. 4
represents a relationship between a development amount of toner and
density at the time of fixing. From FIGS. 1 and 4, it was confirmed
that a density of 1.45 (Macbeth RD-918) generally required in
office documents is obtained by setting the peripheral velocity
ratio to 120%. It was found that, by further increasing the
peripheral velocity ratio, a density of 1.72 is reached at a
peripheral velocity ratio of 200% and, while the density
subsequently continues to vary, an amount of variation is not
large. In consideration thereof, in the present embodiment, as a
first peripheral velocity ratio, a peripheral velocity ratio in the
normal mode (mode A) intended for office applications and the like
is set to 120% at which a density of 1.45 is output. For example,
when the peripheral velocity of the photosensitive drum is 200
mm/sec, the peripheral velocity of the developing roller is 240
mm/sec. In addition, as a second peripheral velocity ratio, a
peripheral velocity ratio in the high-density mode (mode B)
according to the present embodiment is set to 240% as a condition
under which development residual toner is created while producing a
density of 1.7 or higher. For example, when the peripheral velocity
of the photosensitive drum is 200 mm/sec, the peripheral velocity
of the developing roller is 480 mm/sec. Moreover, in the present
embodiment, since the photosensitive drum 201 and the developing
roller 302 rotate in a same direction in the contact portion C1,
the peripheral velocity ratio takes a positive value. Therefore, in
an apparatus configuration in which the photosensitive drum 201 and
the developing roller 302 rotate in opposite directions in the
contact portion C1, the peripheral velocity ratio takes a negative
value. In the present embodiment, a peripheral velocity ratio is
obtained with reference to a contact portion at which the
photosensitive drum and the developing roller are in contact with
each other. However, this method is not restrictive and, in the
case of an apparatus configuration in which the photosensitive drum
and the developing roller do not come into contact with each other,
a position corresponding to a distance of closest approach between
the photosensitive drum and the developing roller may be considered
an opposing portion and a rotation direction may be specified and a
peripheral velocity ratio may be calculated with reference to the
opposing portion. It was confirmed that, under the condition
described above, a state where toner remains on the developing
roller is created even immediately after a development operation of
a high printing pattern such as a solid black image. It was also
confirmed that a density of 1.75 which is sufficiently high is
outputted and M/S of toner remaining on the developing roller
immediately after the development operation was approximately
0.4.times.10.sup.-3 kg/m.sup.2. On the other hand, as a comparative
example, when the peripheral velocity ratio in the high-density
mode is set to .DELTA.v=200% which is a conventional peripheral
velocity ratio at which no development residue is created, it was
confirmed that a state where there is no residual toner on the
developing roller immediately after a development operation is
created when forming a high printing pattern such as a solid black
image while obtaining a density of around 1.72.
[0071] In each mode, 50 sheets of A4 paper were used to
consecutively print a full-size solid black image and the presence
or absence of variations in solid density or non-uniformity during
the printing was confirmed. In addition, solid density was measured
at four corners of a sheet of A4 paper using a density measuring
instrument. A "good" score was given when image density
non-uniformity within the sheet was lower than 0.1 due to low
visibility, otherwise a "not good" score was given. Results thereof
are shown in Table. 1.
TABLE-US-00001 TABLE 1 RELATIONAL DEVELOPMENT 1ST 10TH 20TH 30TH
40TH 50TH DENSITY EXPRESSION RESIDUE SHEET SHEET SHEET SHEET SHEET
SHEET MODE A 1.43~1.46 Q/S.DELTA.v .ltoreq. C.DELTA.V ABSENT GOOD
GOOD GOOD GOOD GOOD GOOD EMBODIMENT 1.74~1.76 Q/S.DELTA.v >
C.DELTA.V PRESENT GOOD GOOD GOOD GOOD GOOD GOOD MODE B COMPARATIVE
1.41~1.74 Q/S.DELTA.v .ltoreq. C.DELTA.V ABSENT GOOD GOOD NOT NOT
NOT NOT EXAMPLE MODE B GOOD GOOD GOOD GOOD
[0072] Table 1 shows a presence or absence of development residue
together with relational expressions. As shown in Table 1, in the
embodiment, across results of 50 consecutive sheets, a level of
image density non-uniformity within pages was favorable while
density was maintained in each of the pages. In contrast, in a
comparative example which is a conventional example, image density
non-uniformity was observed at a rear end portion of the images
starting from the 20th sheet. The level of image density
non-uniformity was not a level of blank dots at which the image
completely disappears but rather in a state resembling a hazy image
in which the rear end portion had a density ranging from around 1.4
to around 1.6 as compared to an overall density of 1.7.
[0073] According to the present embodiment, when forming a solid
image, by adopting an operating condition under which development
residual toner is formed instead of an operating condition under
which development residual toner is not formed as was conventional,
density stability can be obtained in a case where image formation
is consecutively performed. It is conceivable that, by increasing a
toner supply amount and retaining, as development residue toner, a
part of toner to which charges are imparted in the process of
adhering to the developing roller, adherence of toner attracted by
the charge of the development residue toner to the developing
roller proceeds to inhibit the creation of image density
non-uniformity. Moreover, in the development setting which allows
development residue toner to be formed according to the present
embodiment, image density non-uniformity due to a fluctuation in
applied bias described earlier is a concern. However, as shown in
FIG. 4, even when M/S fluctuation occurs in a high-density region,
since density fluctuation is small, an effect of a fluctuation in
applied bias on density change is small.
[0074] As described above, in the present embodiment, when adopting
a high-density mode or a mode intended to increase tinge, the CPU
215 which is a controller adjusts the peripheral velocity ratio
.DELTA.v based on a relationship expressed as
|Q/S.times..DELTA.v|=|C.times..DELTA.V| and forms development
residual toner on the developing roller. Accordingly, even a
high-density image can be printed in a stable manner without
creating image density non-uniformity. Note that the various
operation settings described in the present embodiment are merely
examples. What is important is whether or not a development
contrast formed by a light-part potential of a photosensitive drum
and a developing bias can be completely eliminated by toner having
electrified charges during development (whether or not development
residual toner can be formed) and, as long as this holds true,
other setting conditions may be adopted.
[0075] [Description of Enlargement of Color Gamut]
[0076] FIG. 5 is a chromaticity diagram comparatively showing a
color gamut when forming a color image in the normal mode and a
color gamut when forming a color image in the high-density mode.
The L*a*b* color system (CIE) was used to assess the color gamuts.
In addition, chromaticity was measured using Spectordensitometer
500 manufactured by X-Rite, Incorporated. FIG. 5 shows a change in
color gamuts when control in the high-density mode according to the
present invention is performed in a same manner on the respective
process cartridges of yellow (Y), magenta (Mg), and cyan (Cy) which
constitute basic colors in color image formation. It is shown that,
by switching from the normal mode to the high-density mode, for
example, a color gamut of red (R) formed by yellow (Y) and magenta
(Mg) and a color gamut of green (G) formed by yellow (Y) and cyan
(Cy) have been enlarged.
[0077] Moreover, the high-density mode according to the present
invention is also applicable when only a color gamut of a specific
tinge is enlarged. For example, when only enlarging a color gamut
of blue (B) formed by magenta (Mg) and cyan (Cy), the high-density
mode according to the present invention may be performed only on
the process cartridges of magenta and cyan among the four process
cartridges. Accordingly, an enlargement of a color gamut of a
specific tinge can be more reliably realized without causing a
shortage of a toner supply amount. In addition, the present
invention is also applicable to cases of controlling tinge
adjustment so that rates of increasing a toner laid-on level per
unit area are differentiated among process cartridges. In other
words, by performing the control according to the present invention
when performing the high-density mode in order to adjust a ratio of
toner laid-on levels per unit area between process cartridges to a
prescribed ratio, the prescribed ratio can be more reliably
realized without causing a shortage of a toner supply amount. As a
result, finer adjustment of tinge can be reliably performed.
Second Embodiment
[0078] In the first embodiment described above, when performing a
high-density mode, an operating condition which enables toner to
remain on a developing roller even during printing of a high
printing pattern such as solid black is adopted to realize
stabilization of density and tinge while maintaining supplying
performance of toner to the developing roller. A second embodiment
of the present invention is configured such that, when an
electrified charge amount of toner changes due to a change in the
toner accompanying environmental conditions or specifications,
peripheral velocity ratio control corresponding to the change is
performed so that similar effects are obtained regardless of
changes in conditions and the like. Specifically, an image forming
apparatus according to the second embodiment includes a sensor 219
as detecting means configured to detect temperature and humidity
(refer to FIG. 2). In addition, a CPU 315 assumes a low
temperature, low humidity environment when the temperature detected
by the sensor 219 is equal to or lower than a prescribed
temperature and the humidity detected by the sensor 219 is equal to
or lower than a prescribed humidity and performs control necessary
in a low temperature, low humidity environment to be described
later. On the other hand, the CPU 315 assumes a high temperature,
high humidity environment when the temperature detected by the
sensor 219 is equal to or higher than a prescribed temperature and
the humidity detected by the sensor 219 is equal to or higher than
a prescribed humidity and performs control necessary in a high
temperature, high humidity environment to be described later.
Moreover, in the second embodiment, only differences from the first
embodiment will be described. Matters not described in the second
embodiment are similar to those described in the first
embodiment.
[0079] First, in the configuration of the first embodiment
described above, the presence or absence image density
non-uniformity and of development residue were confirmed in a high
temperature, high humidity environment in which an electrified
charge of toner is less readily obtained and in a low temperature,
low humidity environment in which an electrified charge of toner is
more readily obtained. Table 2 represents a result thereof.
TABLE-US-00002 TABLE 2 AVERAGE DEVELOPMENT 1ST 10TH 20TH 30TH 40TH
50TH DENSITY RESIDUE SHEET SHEET SHEET SHEET SHEET SHEET 1ST
EMBODIMENT 1.75 PRESENT GOOD GOOD GOOD GOOD GOOD GOOD NORMAL
TEMPERATURE, NORMAL HUMIDITY LOW TEMPERATURE, 1.65 SIGNIFICANT GOOD
GOOD GOOD GOOD GOOD GOOD LOW HUMIDITY HIGH TEMPERATURE, 1.77 ABSENT
GOOD GOOD NOT NOT NOT NOT HIGH HUMIDITY GOOD GOOD GOOD GOOD
[0080] In the present embodiment: an environment with a temperature
of 25.degree. C. and a humidity of 60% RH is assumed as a normal
temperature, normal humidity environment; an environment with a
temperature of 15.degree. C. and a humidity of 10% RH is assumed as
the low temperature, low humidity environment; and an environment
with a temperature of 30.degree. C. and a humidity of 80% RH is
assumed as the high temperature, high humidity environment.
Thresholds for determining the low temperature, low humidity
environment are set to a temperature of 20.degree. C. (first
threshold temperature) and a humidity of 30% RH (first threshold
humidity) and, when detected values are equal to or lower than
20.degree. C. and equal to or lower than 30% RH, the CPU 315
determines that an apparatus environment is the low temperature,
low humidity environment. In addition, thresholds for determining
the high temperature, high humidity environment are set to a
temperature of 28.degree. C. (second threshold temperature) and a
humidity of 70% RH (second threshold humidity) and, when detected
values are equal to or higher than 28.degree. C. and equal to or
higher than 70% RH, the CPU 315 determines that an apparatus
environment is the high temperature, high humidity environment.
Moreover, boundaries of temperature and humidity which affect an
electrified charge of toner are to be changed as appropriate in
accordance with a material of the toner, apparatus configuration,
and the like. As shown in Table 2, in the high temperature, high
humidity environment, while the density was around 1.7 which is
slightly higher than in the normal temperature, normal humidity
environment, an amount of development residual toner was depleted
and image density non-uniformity occurred. In addition, in the low
temperature, low humidity environment, although image density
non-uniformity did not occur, the density was around 1.65 which is
slightly lower than in the normal temperature, normal humidity
environment.
[0081] M/S and Q/S on the developing roller are similarly
confirmed. Table 3 represents a result thereof.
TABLE-US-00003 TABLE 3 M/S[kg/m.sup.2] Q/S[C/m.sup.2] Q/M[C/kg] 1ST
EMBODIMENT 3.0 .times. 10.sup.-3 -0.15 .times. 10.sup.-3 -0.050
.times. 10.sup.-3 NORMAL TEMPERATURE, NORMAL HUMIDITY LOW
TEMPERATURE, 3.0 .times. 10.sup.-3 -0.20 .times. 10.sup.-3 -0.067
.times. 10.sup.-3 LOW HUMIDITY HIGH TEMPERATURE, 3.0 .times.
10.sup.-3 -0.14 .times. 10.sup.-3 -0.047 .times. 10.sup.-3 HIGH
HUMIDITY
[0082] Table 3 shows that, compared to the normal temperature,
normal humidity environment, while M/S is unchanged, Q/S has
changed. In other words, an electrified charge amount (hereinafter,
denoted by Q/M which is expressed by Q/M=(Q/S)/(M/S)) per unit
weight of toner has changed. Specifically, Q/M decreases in the
high temperature, high humidity environment and increases in the
low temperature, low humidity environment. This represents a result
of the change described above causing Q/S in the relational
expression |Q/S.times..DELTA.v|=|C.times..DELTA.V| to change as
described in the first embodiment which, in turn, causes changes in
a developable toner amount and a residual amount on the developing
roller immediately after a development operation. In consideration
thereof, the development contrast is optimized so as to maintain
density and a residual toner amount on the developing roller
immediately after a development operation at constant levels even
when the environment changes and, accordingly, Q/M of toner
changes.
[0083] As described in the first embodiment, a development contrast
in the normal temperature, normal humidity environment is
.DELTA.V=200 V as a first development contrast. Meanwhile, when
calculating .DELTA.V based on the change in Q/S and the relational
expression |Q/S.times..DELTA.v|=|C.times..DELTA.V|, .DELTA.V in the
high temperature, high humidity environment as a third development
contrast is calculated as 180 V which is lower than that in the
normal temperature, normal humidity environment. In a similar
manner, .DELTA.V in the low temperature, low humidity environment
as a second development contrast is calculated as 260 V. In the
present embodiment, the development contrasts .DELTA.V under the
conditions described above were adjusted by finely adjusting a
laser light amount. Specifically, the development contrasts
.DELTA.V were adjusted to desired values by fixing the developing
bias to -300 V and the dark-part potential to -500 V (in other
words, fixing the charging bias) and changing the light-part
potential which changes with an increase or decrease of the laser
light amount from -100 V in the normal temperature, normal humidity
environment. Results thereof are shown in Table. 4. Moreover, the
development contrast .DELTA.V may be changed by adjusting the
developing bias or the charging bias instead of adjusting the laser
light amount or by adjusting the developing bias or the charging
bias in addition to adjusting the laser light amount.
TABLE-US-00004 TABLE 4 AVERAGE DEVELOPMENT 1ST 10TH 20TH 30TH 40TH
50TH DENSITY RESIDUE SHEET SHEET SHEET SHEET SHEET SHEET 1ST
EMBODIMENT 1.75 PRESENT GOOD GOOD GOOD GOOD GOOD GOOD NORMAL
TEMPERATURE, NORMAL HUMIDITY LOW TEMPERATURE, 1.74 PRESENT GOOD
GOOD GOOD GOOD GOOD GOOD LOW HUMIDITY HIGH TEMPERATURE, 1.75
PRESENT GOOD GOOD GOOD GOOD GOOD GOOD HIGH HUMIDITY
[0084] Table 4 shows that, by finely adjusting the laser light
amount and changing the development contrast, development residual
toner can be formed in a similar manner to the normal temperature,
normal humidity environment. As an effect thereof, it was confirmed
that an occurrence of image density non-uniformity can be
suppressed in addition to suppressing changes in density.
[0085] While the present embodiment describes that, when an
electrified charge amount (Q/M) of toner changes in accordance with
a fluctuation in the environment, a similar effect is produced with
respect to density and image density non-uniformity by
appropriately adjusting the development contrast with a laser light
amount, this method is not restrictive. As described above, as
means for adjusting the development contrast, for example, a
similar effect may be produced by adjusting a charging bias or a
developing bias. In addition, a cause of a change in the
electrified charge amount (Q/M) of toner is not limited to a change
in the environment and may also be caused by, for example, a
frequency of use. Therefore, a similar effect can be produced by
taking similar actions in accordance with a change in the frequency
of use.
Third Embodiment
[0086] FIG. 7 is a schematic sectional view of an image forming
apparatus according to a third embodiment of the present invention.
The image forming apparatus 200 according to the third embodiment
includes, in addition to the configuration of the image forming
apparatus according to the first embodiment, a recording material
type detection sensor 220 which detects a width of the recording
material 207 in a sub-scanning direction and a recording material
type detection sensor 221 which detects a width of the recording
material 207 in a main scanning direction. In the third embodiment,
only differences from the first and second embodiments will be
described. Matters not described in the third embodiment are
similar to those described in the first and second embodiments.
[0087] FIG. 8 is a block diagram showing driving and high-voltage
control of the photosensitive drum 201 and the developing roller
302 of the image forming apparatus according to the third
embodiment. In FIG. 8, signals are sent from the CPU 215 of the
engine controller 214 to the respective voltage applying portions
401 and 402 and the respective motor drive portions 403 and 404.
First, by sending a signal to the charging voltage applying portion
401 to cause DC voltage to be applied to the charging roller 202
and inducing discharge between the charging roller 202 and the
photosensitive drum 201, the CPU 215 forms a uniform dark-part
potential (Vd) on the surface of the photosensitive drum 201. A
laser 217 spot pattern which is emitted from the laser 217 in the
scanner unit 203 in correspondence with image data exposes the
photosensitive drum 201, and a potential of an exposed portion
drops and assumes a light-part potential (Vl). Next, the CPU 215
sends a signal to the developing voltage applying portion 402 to
cause DC voltage to be applied to the developing roller 302, and
causes the toner 305 to be transferred to the light-part potential
(Vl) of the photosensitive drum 201. At this point, signals have
been sent from the CPU 215 to the photosensitive drum motor drive
portion 404 and the developing roller motor drive portion 403 to
drive the photosensitive drum 201 and the developing roller 302 at
a prescribed number of rotations. A type of the recording material
207 is detected by the CPU 215 based on detection values of the
sensors (220 and 221) which detect recording material widths in the
main scanning and sub-scanning directions of the recording material
207.
[0088] FIG. 9 is a characteristic diagram showing a relationship
between a peripheral velocity ratio which is a ratio of a
peripheral velocity of the developing roller 302 to a peripheral
velocity of the photosensitive drum 201 and a toner supply amount
[kg/m.sup.2] from the developing roller 302 to the photosensitive
drum 201 when printing a solid black image, according to the third
embodiment. FIG. 9 shows that, when the peripheral velocity ratio
is increased, the toner amount supplied from the developing roller
302 to the photosensitive drum 201 increases.
[0089] FIG. 10 is a characteristic diagram showing a relationship
between the toner supply amount [kg/m.sup.2] from the developing
roller 302 to the photosensitive drum 201 and image formation
density. FIG. 10 shows that, when the toner amount supplied from
the developing roller 302 to the photosensitive drum 201 is
increased, the density during image formation increases.
[0090] FIG. 11 is a characteristic diagram showing a relationship
between the toner supply amount [kg/m.sup.2] from the developing
roller 302 to the photosensitive drum 201 when the development
contrast is variable and an electrified charge amount [C/m.sup.2]
of toner developed on the photosensitive drum 201. FIG. 11 shows
the result in case a toner with an electrified charge amount -0.05
[C/m.sup.2] is used. FIG. 11 shows that a setting is adopted such
that, when the development contrast is 200 V, a development
efficiency at point A is 100% or, in other words, all of the toner
supplied from the developing roller to the photosensitive drum is
used to develop an electrostatic image on the photosensitive drum.
Beyond point A, the development efficiency becomes lower than 100%
or, in other words, apart of the toner supplied from the developing
roller to the photosensitive drum is not used to develop an
electrostatic image. Settings are respectively adopted such that,
when the development contrast is 250 V and 300 V, the development
efficiency at points B and C is 100% and, beyond points B and C,
the development efficiency becomes lower than 100%.
[0091] In other words, for example, when the development contrast
is 200V, even if the toner supply amount [kg/m.sup.2] from the
developing roller 302 to the photosensitive drum 201 is further
increased from point A, latent image charges on the photosensitive
drum 201 are filled up before all of the toner is used up.
Therefore, a part of the supplied toner is not transferred onto the
photosensitive drum 201 (not used for development) and the
development efficiency declines (becomes lower than 100%
development efficiency) in the relationship of an amount used for
development with respect to a supply amount. Moreover, when the
development contrast is 250 V and 300 V, when the toner supply
amount to the photosensitive drum 201 is similarly increased beyond
points B and C, the development efficiency declines in a similar
manner.
[0092] The development contrast is formed by the light-part
potential (Vl) on the photosensitive drum 201 and a prescribed DC
bias (developing bias) applied to the developing roller 302 and
decreases as toner is transferred from the developing roller 302 to
the photosensitive drum 201. When all of the toner borne by the
developing roller 302 is transferred to the photosensitive drum 201
and used for development and a charge amount [C/m.sup.2] of the
toner causes the development contrast to drop to 0 V, the
development efficiency becomes 100%. In this case, a desired value
of the development contrast is set using a supply amount
[kg/m.sup.2] of the toner 305 from the developing roller 302 to the
photosensitive drum 201 in accordance with a target density, a
charge amount [C/m.sup.2] of the toner developed on the
photosensitive drum 201, and the like. Various operation settings
when the image forming apparatus according to the third embodiment
performs image formation in the normal mode (normal image formation
mode) which is a normal operation will be shown below.
TABLE-US-00005 TABLE 5 DEVEL- DEVEL- PERIPHERAL OPMENT OPMENT
VELOCITY EFFI- CONTRAST RATIO CIENCY TARGET MODE [V] [%] [%]
DENSITY NORMAL 200 145 100 1.35 MODE
[0093] The operation settings in the normal mode shown in Table 5
are calculated as follows. For example, let us assume a target
density setting of 1.35. In this case, FIG. 10 shows that, in order
to obtain a density of 1.35, a toner supply amount from the
developing roller 302 to the photosensitive drum 201 of 0.003
kg/m.sup.2 is required. In addition, FIG. 9 shows that, in order to
realize this toner supply amount, the developing roller 302 must be
driven by the developing roller motor drive portion 403 so that the
peripheral velocity ratio which represents the ratio of the
peripheral velocity of the developing roller 302 to the peripheral
velocity of the photosensitive drum 201 is 145%. Furthermore, FIG.
11 shows that the development contrast is set to 200 V in order to
set the development efficiency to 100% in the toner supply amount
described above.
[0094] According to the settings described above, in a high
printing pattern such as solid black, a sufficient electrostatic
latent image with respect to a charge amount of the toner 305 borne
by the developing roller 302 can be formed and a development
efficiency of 100% which enables all of the toner 305 coating the
developing roller 302 to be transferred to the photosensitive drum
201 can be realized. Therefore, since residual toner of the coat of
toner on the developing roller 302 after a development operation is
almost depleted, when using a paper size or the like which
necessitates consecutive image formation such as long paper, there
is a concern that poor following performance (image density
non-uniformity, color non-uniformity, or the like) may occur at
rear ends of the sheets of paper. In consideration thereof, the
third embodiment is configured such that, together with a normal
image formation mode which realizes normal image quality used in
offices or the like, a plurality of image formation modes which are
selectable in accordance with use conditions of the user can be
set. In the present third embodiment, as the plurality of image
formation modes selectable by the user, a long paper mode is
provided which is an operating mode when consecutively performing
image formation on a plurality of sheets of long paper or, in other
words, a plurality of sheets of recording material having a size in
which a length in a recording material conveyance direction is
relatively long.
[0095] FIG. 6 is a flow chart during an image forming operation
according to the third embodiment. In FIG. 6, the user selects an
image formation mode (instructs the image forming apparatus to
execute an image forming operation) (102). When the user does not
select a mode (103: No), the CPU 215 selects the normal image
formation mode (104) and starts an image forming operation (105).
When the user selects the long paper image formation mode (103:
Yes), the CPU 215 starts image formation in the long paper image
formation mode (106 and 107). Hereinafter, operation settings of
image formation when the user selects the long paper image
formation mode according to the third embodiment will be shown.
TABLE-US-00006 TABLE 6 DEVEL- DEVEL- PERIPHERAL OPMENT OPMENT
VELOCITY EFFI- CONTRAST RATIO CIENCY TARGET MODE [V] [%] [%]
DENSITY NORMAL 200 145 100 1.35 MODE LONG 200 193 75 1.35 PAPER
MODE
[0096] The operation settings in the long paper mode shown in Table
6 are calculated as follows. The poor following performance at the
rear ends of the sheets of long paper which occurs when
consecutively performing image formation on a plurality of sheets
of long paper is caused when the development efficiency is set to
100% and all of the toner 305 coating the developing roller 302 is
transferred to the photosensitive drum 201. In other words, in a
latter half of consecutive image formation, the supply of the toner
305 from the developing roller 302 to the photosensitive drum 201
becomes unfollowable. Therefore, in the long paper mode described
above, a toner coating amount on the developing roller 302 must be
increased by increasing the peripheral velocity ratio between the
photosensitive drum 201 and the developing roller 302 to make the
supply of the toner 305 from the developing roller 302 to the
photosensitive drum 201 followable.
[0097] In this case, in order to improve following performance with
respect to the toner supply from the developing roller 302 to the
photosensitive drum 201, the development efficiency must be set
lower than 100%. By setting the development efficiency lower than
100%, the toner 305 remains on the developing roller 302
immediately after a development operation and produces an effect in
which, due to a charge amount of the remaining toner 305, a larger
proportion of the toner 305 supplied from the supplying roller 304
to the developing roller 302 can be attracted to the developing
roller 302. Accordingly, the toner coating amount of the developing
roller 302 can be maintained over a longer period during
consecutive image formation and following performance with respect
to the toner supply from the developing roller 302 to the
photosensitive drum 201 can be improved.
[0098] To this end, in order to set the development efficiency
lower than 100% and keep development residual toner on the
developing roller 302 while satisfying following performance with
respect to the toner 305 from the developing roller 302, a ratio
(peripheral velocity ratio) of the peripheral velocity of the
developing roller 302 to the peripheral velocity of the
photosensitive drum 201 is increased. An amount by which the
peripheral velocity ratio is increased is obtained as follows. For
example, when the development efficiency is set to 75% as a setting
of the development efficiency lower than 100%, FIG. 11 shows that
the toner supply amount from the developing roller 302 to the
photosensitive drum 201 in order to realize the development
efficiency of 75% is 0.004 kg/m.sup.2. In addition, FIG. 9 shows
that the peripheral velocity ratio of the developing roller 302 to
the photosensitive drum 201 necessary to realize the toner supply
amount is 193%. Therefore, it is shown that the CPU 215 which is a
controller need only drive the developing roller 302 with the
developing roller motor drive portion 403 so that the peripheral
velocity ratio is attained.
[0099] By setting the long paper mode described above, when using
long paper with a width (length) in the sub-scanning direction of
1200 mm, poor following performance (image density non-uniformity,
color non-uniformity, or the like) in rear ends of the sheets of
paper which occurs in the normal mode can be suppressed. Moreover,
the normal mode is an operating mode which assumes image formation
to be mainly performed on recording material with regular sizes
such as A5 and A4.
[0100] As described above, in the third embodiment, together with
an image formation mode which realizes normal image quality, a long
paper mode is provided as one of a plurality of image formation
modes which are selectable in accordance with use conditions of the
user. In addition, by adopting the operation settings according to
the third embodiment described above, when consecutively performing
image formation on a plurality of sheets of long paper, an
occurrence of poor following performance (image density
non-uniformity, color non-uniformity, or the like) in rear ends of
the sheets of paper which sometimes occurs in the normal image
formation mode can be suppressed. While the third embodiment is
configured so that the long paper mode can be selected by the user,
the configuration is not restrictive as long as a similar effect
can be produced. For example, the image forming apparatus 200
itself may detect a paper type and the CPU 215 may automatically
select the long paper mode.
[0101] While the third embodiment adopts a configuration in which a
developing roller motor drive portion and a photosensitive drum
motor drive portion are respectively shared or, in other words,
four developing rollers are rotated by a single motor and four
photosensitive drums are rotated by another single motor, a driving
configuration is not limited thereto. As long as the operation
settings in the long paper mode described above can be realized,
for example, a configuration in which each developing roller and
each photosensitive drum are respectively rotationally driven by
independent motors can be adopted.
[0102] In addition, while a peripheral velocity ratio of the
photosensitive drum 201 and the developing roller 302 is set by
changing the number of rotations of the developing roller 302, the
peripheral velocity ratio may be varied by changing the number of
rotations of the photosensitive drum 201 while keeping the number
of rotations of the developing roller 302 fixed. Alternatively, the
peripheral velocity ratio may be variably controlled by changing
both the number of rotations of the developing roller 302 and the
number of rotations of the photosensitive drum 201. In this case,
the peripheral velocity ratio may be changed by reducing both the
number of rotations of the developing roller 302 and the number of
rotations of the photosensitive drum 201 while differentiating
amounts of reduction thereof.
[0103] In addition, while a development contrast of 200 V, a
peripheral velocity ratio of 193%, and a development efficiency of
75% are adopted in the third embodiment as operation setting values
in the long paper mode, appropriate setting values may naturally
differ in accordance with apparatus configurations, operating
conditions, or the like. The respective setting values may be
changed as appropriate as long as similar effects to the third
embodiment can be produced.
Fourth Embodiment
[0104] In the third embodiment, a long paper mode for accommodating
long paper while maintaining similar values to a normal mode with
respect to density is described as one of a plurality of image
formation modes for increasing a tinge selection range or obtaining
high density. In contrast, as yet another operating mode which is
selectable by the user, a fourth embodiment of the present
invention is provided with a high-density mode for increasing a
tinge selection range and/or obtaining high density. In the
high-density mode, various operation settings are designed to
prevent the occurrence of poor following performance (image density
non-uniformity, color non-uniformity, or the like). In the fourth
embodiment, only differences from the third embodiment will be
described. Matters not described in the fourth embodiment are
similar to those described in the third embodiment.
[0105] FIG. 12 is a flow chart during an image forming operation
according to the fourth embodiment of the present invention. In
FIG. 12, the user selects an image formation mode (instructs the
image forming apparatus to execute an image forming operation)
(802). When mode selection is not performed (803: No and 804: No),
the CPU 215 starts an image forming operation in the normal image
formation mode (805 and 806). When the user does not select the
long paper image formation mode but selects the high-density image
formation mode (803: No and 804: Yes), the CPU 215 starts image
formation in the normal high-density image formation mode for
regular size paper (807 and 808). When the user selects the long
paper image formation mode and also selects the high-density image
formation mode (803: Yes and 812: Yes), the CPU 215 starts image
formation in the high-density image formation mode for long paper
(813 and 814). When the user selects the long paper image formation
mode but does not select the high-density image formation mode
(803: Yes and 812: No), the CPU 215 starts image formation in the
long paper image formation mode described in the third embodiment
(809 and 810). Hereinafter, operation settings of image formation
when the user selects the high-density image formation mode
according to the fourth embodiment will be shown.
TABLE-US-00007 TABLE 7 DEVEL- DEVEL- PERIPHERAL OPMENT OPMENT
VELOCITY EFFI- CONTRAST RATIO CIENCY TARGET MODE [V] [%] [%]
DENSITY NORMAL 200 145 100 1.35 MODE LONG PAPER 200 193 75 1.35
MODE NORMAL 300 360 93 1.75 HIGH-DENSITY MODE LONG PAPER 300 483 75
1.75 HIGH-DENSITY MODE
[0106] The operation settings in the high-density modes shown in
Table 7 are calculated as follows. For example, in the normal
high-density mode, a target density thereof is set to 1.75 in
consideration of market needs. Accordingly, FIG. 10 shows that, in
order to satisfy the target density of 1.75, a toner supply amount
from the developing roller 302 to the photosensitive drum 201 of
0.007 kg/m.sup.2 is required. In addition, FIG. 11 shows that, in
order to transfer 0.007 kg/m.sup.2 of toner from the developing
roller 302 to the photosensitive drum 201, a development contrast
of 300 V is required. In this case, in order to print a
high-density image without causing an occurrence of poor following
performance (image density non-uniformity, color non-uniformity, or
the like), the development efficiency must be set lower than 100%.
In consideration thereof, the supply amount of the toner 305 from
the developing roller 302 to the photosensitive drum 201 is set to
0.0075 kg/m.sup.2. In order to set the supply amount of the toner
305 from the developing roller 302 to the photosensitive drum 201
to 0.0075 kg/m.sup.2, FIG. 9 shows that the peripheral velocity
ratio of the developing roller 302 to the photosensitive drum 201
must be set to 360%. Therefore, the developing roller 302 is driven
by the developing roller motor drive portion 403 so that the
peripheral velocity ratio of 360% is attained. As a result, the
development efficiency becomes 93%. On the other hand, in the
high-density mode for long paper, in order to attain the same
target density of 1.75 as the normal high-density mode, the
peripheral velocity ratio is increased to set the development
efficiency lower than 100% for the purpose of improving following
performance with respect to toner supply at a development contrast
of 300V. Specifically, the peripheral velocity ratio is increased
to 483% and the development efficiency is lowered to 75%.
[0107] By setting the high-density mode described above, when
performing image formation on long paper, the density can be
increased from 1.35 to 1.75 without causing an occurrence of poor
following performance (image density non-uniformity, color
non-uniformity, or the like) and a preferable high-density image
can be obtained. In other words, an enlarged color gamut and an
increased tinge selection range realized by the high-density mode
described in the first embodiment can also be realized using long
paper without causing image defects.
[0108] Moreover, while a density of 1.75, a development contrast of
300 V, a peripheral velocity ratio of 360%, and a development
efficiency of 93% are adopted in the fourth embodiment as operation
setting values in the normal high-density mode, appropriate setting
values may naturally differ in accordance with apparatus
configurations, operating conditions, or the like. In a similar
manner, while a density of 1.75, a development contrast of 300 V, a
peripheral velocity ratio of 483%, and a development efficiency of
75% are adopted in the fourth embodiment as operation setting
values in the high-density mode for long paper, appropriate setting
values may naturally differ in accordance with apparatus
configurations, operating conditions, or the like. The respective
setting values may be changed as appropriate as long as similar
effects to the fourth embodiment can be produced.
[0109] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all such modifications and
equivalent structures and functions.
[0110] This application claims the benefit of Japanese Patent
Application No. 2016-057626, filed on Mar. 22, 2016, which is
hereby incorporated by reference herein in its entirety.
* * * * *